Abstract

Background

Recent translocations of autosomal regions to the sex chromosomes represent important
systems for identifying the evolutionary forces affecting convergent patterns of sex-chromosome
heteromorphism. Additions to the sex chromosomes have been reported in the melanica and robusta species groups, two sister clades of Drosophila. The close relationship between these two species groups and the similarity of their
rearranged karyotypes motivates this test of alternative hypotheses; the rearranged
sex chromosomes in both groups are derived through a common origin, or the rearrangements
are derived through at least two independent origins. Here we examine chromosomal
arrangement in representatives of the melanica and the robusta species groups and test these alternative hypotheses using a phylogenetic approach.

Results

Two mitochondrial and two nuclear gene sequences were used to reconstruct phylogenetic
relationships of a set of nine ingroup species having fused and unfused sex chromosomes
and representing a broad sample of both species groups. Different methods of phylogenetic
inference, coupled with concurrent cytogenetic analysis, indicate that the hypothesis
of independent origins of rearranged sex chromosomes within each species group is
significantly more likely than the alternative hypothesis of a single common origin.
An estimate tightly constrained around 8 My was obtained for the age of the rearranged
sex chromosomes in the melanica group; however, a more loosely constrained estimate of 10–15 My was obtained for the
age of the rearrangement in the robusta group.

Conclusion

Independent acquisition of new chromosomal arms by the sex chromosomes in the melanica and robusta species groups represents a case of striking convergence at the karyotypic level.
Our findings indicate that the parallel divergence experienced by newly sex-linked
genomic regions in these groups represents an excellent system for studying the tempo
of sex chromosome evolution.